\documentclass[a4paper]{article}
\usepackage{tikz}

\usepackage{geometry}
\geometry{left=1cm, right=1cm}

\usetikzlibrary{external} % set up externalization

\tikzset{external/system call={latex \tikzexternalcheckshellescape -halt-on-error
-interaction=batchmode -jobname "\image" "\texsource";
dvips -f -o "\image".ps "\image".dvi;
ps2eps -f "\image.ps"}}
\tikzexternalize


\begin{document}

\begin{tikzpicture}
%\draw (0,0) circle (1cm);
\node at (0,0) {\includegraphics[width=.48\textwidth]{../F11.eps}};
\node at (8.5cm,0) {\includegraphics[width=.48\textwidth]{../F12.eps}};

\node at (0,-6.5cm) {\includegraphics[width=.48\textwidth]{../F13.eps}};
\node at (8.5cm,-6.5cm) {\includegraphics[width=.48\textwidth]{../F14.eps}};

%\node at (0,-13.cm) {\includegraphics[width=.48\textwidth]{../E3.eps}};
%\node at (8.5cm,-13.cm) {\includegraphics[width=.48\textwidth]{../FF3.eps}};
\end{tikzpicture}

\end{document}
